Blender Cycles Slow Rendering Interior Scene
In outdoor scenes most rays don't bounce much and just fly off into the sky.

In indoor scenes almost all the rays will end up reflecting around up to the maximum number of bounces. Not only does this often mean more noise, but also slower rendering (more ray intersection calculations etc.)

You can speed this up a little by reducing the number of bounces. You can also try and reduce noise by using MIS and non-mesh lamps where appropriate.

Related:


Spherical emitter does not illuminate very well?
When should MIS be used and when should it be disabled?
Managing/Reducing noise in a night scene




Note that in 2.75 cycles will have Light Portals, which can be used to guide rays through openings. This is useful for indoor scenes lit by an outside source.

To set up a light portal, add an area lamp which covers an opening (e.g. a window), then enable Portal in the lamp settings:



The direction matters, the lamp should be pointing into the interior.

Portals will actually make sampling slower, but converge the scene with far fewer samples. So much so that if you reduce the sample count, you can get less noise with fewer samples and a shorter render time than if you didn't use portals: